The present invention relates to the processing of grain oriented electrical steel and particularly to a process wherein the glass film formed by reacting an annealing separator with the electrical steel during the final high temperature anneal may be easily removed.
Electrical steel is normally subjected to a decarburizing anneal in order to lower the carbon present in the steel to prevent magnetic aging. An accepted maximum carbon level is about 0.004%. The wet decarburizing atmosphere reduces the iron and oxidizes the carbon and silicon. The carbon is removed in the form of a gaseous oxide and the silicon present in the base metal is oxidized to silica which remains on the surface and as inclusions beneath the surface. The steel is then coated with a magnesia annealing separator and subjected to a high temperature final anneal in which the secondary grain growth is developed. The magnesia reacts with the silica and produces a tightly adherent glass film of magnesium silicate, also known as forsteritc (Mg.sub.2 SiO.sub.4), which provides interlaminar resistivity and prevents the laps of the steel coil from sticking together. It is also very important that the annealing separator does not interfere with purification of the steel during the high temperature anneal.
The presence of the glassy film is not always advantageous for subsequent processing. This hard and abrasive oxide is very hard on punching dies used to stamp out the laminations for producing transformer cores. It is also very difficult to remove the glass by pickling in strong acids or by using abrasive means.
The production of punching quality electrical steel has normally limited the thickness of the glass film formed and subsequently removed the glass by pickling in strong acids. In the past, a coating of 0.5 mm thickness was considered sufficiently thin to be removable.
Previous attempts to limit or reduce the glass film formation, however, have been found to have an adverse impact on the secondary grain growth stability and have resulted in poor magnetic quality (typically incomplete grain growth and/or poor texture development).
U.S. Pat. No. 3,930,906 (Toshio Irie et al.--assigned to Kawasaki Steel Corporation) found that good magnesia adhesion was developed when the iron oxide on the surface during decarbufization oxidized the silicon in the base metal to SiO.sub.2. When the iron oxide was reduced with hydrogen, the film had low adhesion. The patent discusses the role of atmosphere, penetration between the laps of the coil and heating conditions on the formation of the MgO-SiO.sub.2 glass film.
One could use a separator such as alumina which does not interfuse with the silica on the surface, but it is very difficult to desulfurize the steel with this coating on the surface. The adherence doesn't allow for good handling and processing through the annealing stages. Japanese Published Unexamined Patent Application No. 53(1978)-22113 uses an annealing separator consisting of fine alumina powder blended with hydrated silica to suppress the formation of a glass film. The resulting oxide film is very thin.
Prior magnesias were normally active magnesia which had citric acid activities below 200 seconds and typically below 100 seconds. Inactive magnesia was not used because the slurry was not stable and the magnesia particles tended to settle to the bottom of the tank. Calcining the magnesia above 1300.degree. C. reduced its reactivity and suppressed the formation of forsteritc.
There have been very few patents which have attempted to use inactive magnesia to coat grain oriented silicon steel. U.S. Pat. No. 4,344,802 (Michael H. Haselkom--assigned to Armco Inc.) worked with magnesia which had a citric acid activity greater than 200 seconds. Phosphates were added to the magnesia to keep the particles from settling which created a slurry with a viscosity that could be applied to the steel and produce an acceptable coating weight. The resulting slurry had good adherence and reacted with the steel surface to form a glass film
Japanese Published Unexamined Patent Application No. 59(1984)-96278 discloses an annealing separator which consists of Al.sub.2 O.sub.3 which has a low reactivity with the SiO.sub.2 in the oxide film formed during decarburization. Part of the annealing separator is MgO which was calcined at more than 1300.degree. C. to reduce its reactivity. This separator suppresses the formation of forsterite.
U.S. Pat. No. 3,375,144 (David W. Taylor--assigned to Armco Steel Corporation) mixed alkali metals, such as the sulfides and hydroxides of sodium and potassium, with the magnesia to enable the easy removal of the surface by scrubbing and short-time pickling. It was believed that the addition removed sub-surface siliceous particles.
U.S. Pat. No. 3,378,581 (Dale M. Kohler--assigned to Armco Steel Corporation) added calcium oxide to magnesia as the annealing separator to improve desulfurization. The surfaces were to be free of overlying adherent films of annealing separators and glassy derivatives therefrom. Thin films were desired and the formation of a glass film was largely avoided by the use of a nonhydrating magnesia. A thick glass film and one which will be oxidizing to the iron will be avoided by using calcium oxide.
U.S. Pat. No. 4,875,947 (Hisanobu Nakayama et al--assigned to Nippon Steel Corporation) prevents the formation of a glass film by adding one or more salts of alkali metals such as Li, Na, K and alkaline-earth metals such as Ca, Ba, Mg and Sr to the magnesia. The salt decomposes the SiO.sub.2 in the oxide film and prevents the reaction which forms the glass. To maintain the good punching characteristics, an inorganic coating is applied to prevent oxidation during a thermal flattening or stress relief annealing and then an organic coating is applied which improves the punching property.
A decarburizing treatment will thus oxidize the surface of silicon steel and produce at and near the surface a distinct layer of silica. U.S. Pat. No. 3,201,293 (Victor W. Curtis--assigned to Armco Steel Corporation) found that heat treatment in a decarburizing atmosphere will give a satisfactory die life only up to about 1700.degree. F. which is not high enough to develop the optimum magnetic properties. A band or line of oxide at the original interface between the base metal and the skin forms during decarburization. The oxidation of the silicon below the band in the final high temperature anneal raises the band to about the mid thickness of the final surface.
The discussion above clearly illustrates that there is a need for an annealing separator coating for electrical steel which forms a glass which is easily removed. Prior attempts to limit the glass formation have not optimized the magnetic quality or have resulted in glass which is not easily and completely removable. Prior magnesia coating systems have not been directed to the control of the interface between the coating and the base metal in order to provide a coating which is easily removed.